1. Field of the Invention
The present invention generally relates to battery packs. More particularly, the present invention relates to battery packs having battery cells that are disposed within a case and are used as power sources for electric power tools.
2. Description of the Related Art
Generally, battery packs for electric power tools include exposed electric terminals for charging and discharging the enclosed battery cells. When a battery pack is attached to an electric power tool, electrical contact is established between the terminals of the battery pack and the terminals of the battery holder in the tool, thus supplying power to the tool. When the charge level of a battery pack becomes low, the pack is set in a battery charger to establish electrical contact between the charging and discharging terminals of the pack and the charging terminals of the battery charger, thus recharging the battery cells in the pack.
When charging the battery cells in a battery pack, the temperature of the battery cells rises due to buildup of heat in the battery cells themselves and heat conduction from the transformer of the charger. In certain types of cells and batteries, such as nickel metal hydride cells, this phenomenon is so severe that the service life of the cells will be shortened unless some preventative measures are taken. One structure for preventing heat buildup in a battery pack is disclosed in Japanese Laid-open Utility Model Publication No. 6-54209. This structure includes within a charger a fan for delivering air to the terminals and air passages running through the cells in the battery pack and connecting openings provided in the battery pack case. When the battery pack is set in the charger, the fan sends air through the cells along the passages in order to prevent heat buildup in the battery cells.
Although the foregoing structure can significantly lessen the deterioration of the battery cells by preventing heat buildup in the battery cells, it suffers from other drawbacks. For example, moisture and other foreign matter can easily enter the battery pack through the openings in the battery pack and may rust the battery cells, which may cause leakage of the electrolyte. Furthermore, conductive foreign matter may cause a short-circuit. In order to solve problems caused by entry of foreign matter, Japanese Utility Model Publication No. 5-37634 discloses an improved combination of a battery pack and a charger. The battery pack is provided with interlocked doors that open when the pack is set on the charger and close when the pack is removed from the charger so as to keep out foreign matter. One significant drawback of this structure is an increase in the manufacturing cost due to the sliding doors and guide bosses protruding from the charger for opening and closing the doors. Moreover, the existence of the guide bosses limits the types of battery packs that can be recharged on the charger, thus reducing its general versatility. The durability and reliability of the mechanism for operating the sliding doors is somewhat questionable because the relatively fragile guide bosses may break when the battery pack is set on the charger.